A conventional bascule bridge interconnecting sections of a roadway or thoroughfare generally comprises a single-leaf bascule bridge span formed of torsionally rigid box-section girders, which are expensive to construct and maintain, and a massive counterweight disposed on a relatively short lever arm or rearward extension located behind a pivot point. The counterweight reduces torque and power requirements of a machine that raises and lowers the bascule bridge span about the pivot point. In double-leaf bascule bridges, counterweights on opposing bascule bridge spans also reduce forces on an anchoring devices that secure the opposing spans in a lowered position. A bascule bridge span having a counterweight requires a large volume of space or a pit below the roadway, and often below a water surface, to accommodate the counterweight as it swings downward through an arc of travel when the bridge is raised. The pit however represents a significant expense in the construction of a conventional bascule bridge. The size of the counterweight may be reduced by extending the length of the lever arm on which the counterweight is disposed but this requires an even larger pit to accommodate the increased arc traveled by the extended lever arm.
In the past, conventional bascule bridges have generally comprised a steel grating or steel plates disposed on the bascule bridge span as a roadway deck because it was thought that steel was lighter in weight than concrete and less susceptible to stress while raising and lowering the bascule bridge span. Steel grating however provides a rough roadway surface that is particularly noisy when traversed by motorized traffic and is dangerous to pedestrians who may easily loose their footing on the grated surface. Steel grating also allows drippings from vehicles to pass through the roadway deck onto water or land below which has an undesirable effect on the environment. It has been suggested to overcome these problems by filling the steel gratings with concrete. This proposed solution however eliminates the reduced weight benefit of the grating. Moreover, corrosion of the confined steel causes spalling and rapid deterioration of the concrete. Steel plate roadway decks have the disadvantage that they require installation of structural stiffeners which is labor-intensive and costly. Steel plate roadway decks also require an additional wearing surface to prevent corrosion and to improve traction. The wearing surface however must be bonded to the steel plate to ensure integrity during operation of the bridge and usually requires costly fabrication to ensure effective traction. There exists therefore a demonstrated need for an advancement in the art of bascule bridge design.
It is an object of the present invention to provide a novel bascule bridge.
It is also an object of the present invention to provide a novel bascule bridge having an unbalanced bridge span which eliminates the requirement for a large rearward structural extension to support a counterweight and a pit for receiving the counterweight.
It is another object of the present invention to provide a novel bascule bridge that is economical to build and operate.
It is a further object of the present invention to provide a novel unbalanced bascule bridge having longitudinal girders interconnected at a pivoting end by a cross-girder which isolates the longitudinal girders from support and operating forces.
It is yet another object of the present invention to provide a novel lightweight concrete roadway deck that is economical to construct, has a improved durability and service performance, and requires minimum maintenance.
Accordingly, the present invention is directed toward a novel unbalanced bascule bridge having an unbalanced bridge span including a pair of longitudinal girders with a low torsional stiffness interconnected at a pivoting end by a torsionally rigid cross-girder and interconnected along a longitudinal expanse of the longitudinal girders by a steel frame which forms a closely spaced lattice for supporting a relatively thin, lightweight concrete roadway deck. The bascule bridge span is raised and lowered by an actuator assembly including a plurality of hydraulic cylinders pivotally mounted on support columns and corresponding piston rods which apply a torque to the cross-girder through a corresponding pair of crank plates welded to the cross-girder and pivotally interconnected to a base by corresponding main trunnions. When the bascule bridge span is in a cantilevered or raised position, all support forces including the forces of the main trunnions and support members act on the cross-girder through the crank plates and all reaction forces including the forces of the longitudinal girders act directly on the cross-girder. Consequently, the cross-girder effectively isolates the longitudinal girders from the support and operating forces some of which may be non-uniform due to lack of symmetry in the forces applied by the plurality of piston rods, possible bearing or trunnion misalignment and unequal weight distribution by absorbing the non-uniform effects thereby ensuring proper support and alignment of the longitudinal girders.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following Detailed Description of the Invention with the accompanying drawings.